1. Field of the Invention
The present invention relates to a photoelectric conversion device and its manufacturing method, and more particularly to a photoelectric conversion device such as a line sensor or an area sensor for use in reading the image in the OA (Office Automation) equipment such as a facsimile, wherein a photoconductive film is laminated on a monocrystalline semiconductor circuit substrate which is formed with a signal charge storage portion, a signal read circuit, a scan circuit, and a drive circuit, and a manufacturing method which is suitable for the photoelectric conversion device.
2. Related Background Art
Photoelectric conversion elements of the PIN structure or the Schottky structure of using a non-single crystalline semiconductor are widely known, and among them, particularly amorphous semiconductors or microcrystalline semiconductors mainly composed of silicon can be used for the photoelectric conversion unit of a one-dimensional line sensor or a layer-built solid image sensor, because they can be fabricated through the low temperature process and easily in the large area.
In the field of the solid image sensor, there has been a demand for solid image sensors with higher performance and at lower price, and the conventionally used solid image sensors include, for example, a CCD or MOS solid image sensor, wherein in the main stream the peripheral circuits such as a light receiving element portion, a signal charge storage portion, a signal read circuit, a scan circuit, and a signal processing circuit are formed on the same substrate. On the other hand, for the purpose of higher sensitivity by improved opening rate, a layer-built solid image sensor has been proposed (e.g., Japanese Laid-Open Patent Application No. 49-91116 and Japanese Laid-Open Patent Application No. 51-10715) in which a photoconductive film is laminated as light receiving element on the substrate formed with the above-mentioned semiconductor circuit.
And for the purpose of further higher sensitivity, an amplifying photoelectric conversion device and a layer-built solid image sensor have been proposed in Japanese Laid-Open Patent Application No. 3-278482, which make use of collision ionization due to an energy step at the heterojunction of the non-single crystalline semiconductor. FIG. 1A is a schematic cross-sectional view showing the structure of this photoelectric conversion device, FIG. 1B is a typical energy band diagram for this photoelectric conversion device under a non-bias condition, and FIG. 1C is a typical energy band diagram for this photoelectric conversion device under reverse bias. In FIG. 1B, the minimum forbidden band width is indicated by Eg1 and the maximum forbidden band width by Eg2.
As shown in FIG. 1A, a light absorbing layer 810 independent of a multiplying layer and a plurality of graded layers 801 to 809 which become multiplying layers are sandwiched by a p-type semiconductor layer 811 which becomes a charge injection blocking layer and an n-type semiconductor layer 815, wherein the electrical connection is made between the p-type semiconductor layer 811 and an electrode 813, and between the n-type semiconductor layer 815 and an electrode 814, and these layers are formed on a glass substrate 816. The graded layers 801 to 809 have the forbidden band width gradually changing from Eg1 to Eg2, such that the minimum forbidden band width Eg1 and the maximum forbidden band width Eg2 are in contact with each other in adjacent graded layers.
In operation, by applying a voltage at which the carrier drift occurs sufficiently, as shown in FIG. 1C, electrons among carriers occurring in the light absorbing layer 810 due to light incident are drifted to a graded layer 801 where the forbidden band width continuously changes. Drifted electrons reach at a heterojunction region between the maximum forbidden band and the minimum forbidden band where there is an energy step, giving rise to carrier multiplication owing to collision ionization. That is, this photoelectric conversion device amplifies the light signal with lower noise, using collision ionization of photocarriers due to energy step at the heterojunction, and has a super high sensitivity as compared with the photoelectric conversion devices as heretofore known.
A photoelectric conversion device as proposed in the Japanese Laid-Open Patent Application No. 3-278482 as above cited makes use of energy step occurring at the heterojunction of amorphous SiC and amorphous SiGe for the photocarrier multiplication. If an electric field to cause photocarriers to run is applied to such heterojunction, the dark current caused by the defect on the hetero interface may increase in some cases, for which the improvement is desired.
Though the manufacturing method of this hetero interfacial region has not been specifically disclosed, the typical interface forming methods by plasma CVD may include a method in which after deposition of amorphous SiGe, the plasma is stopped temporarily to replace the source gas, and then the plasma is excited again to start the deposition of amorphous SiC, and a method in which the source gas is only changed instantaneously under the condition where the plasma is continuously excited.
However, since the examination by the present inventors indicates that there is no significant difference in the dark current between such methods, it is considered that the mixture of C and Ge especially at the hetero interface is a main factor of producing the dark current.
FIG. 2 shows the results of evaluation with SIMS (Secondary Ion Mass Spectrometry) for the mutual diffusion of Ge and C at the interface between amorphous SiGe and amorphous SiC fabricated by a method in which after the deposition of amorphous SiGe, the plasma is stopped temporarily to replace the source gas, and then the plasma is excited again to start the deposition of amorphous Si. From this results, the interface steepness from an interface width from 84% to 16% can be obtained to be about 40 angstroms. However, since the knock-on of primary ions spreads as far as about 20 angstroms with this spectrometry method, it is believed that the practical mutual diffusion region is about 20 angstroms, and this region will possibly produce the defect due to the mixture of C and Ge. Accordingly, to reduce further dark current, it is necessary that this region of mutual diffusion is made as small as possible.